Water, wastewater and industrial liquid granular filtration units typically have a filter media support system that separates the filter media from the underdrain system and filter bottom. The underdrain system is the primary support for the filter media, and also serves to collect the filtrate and provide for the uniform distribution of air and water during the backwash of the filter system.
Underdrain systems are often made of concrete blocks having spaces to allow for piping, such as air laterals, that are part of the backwash air distribution system. A precast concrete, plastic-jacketed underdrain block is disclosed in U.S. Pat. No. 4,923,606. Nozzle-less type underdrain systems with large openings for the passage of the filtrate and the backwash water are preferred because they do not plug as easily as nozzle type underdrains. Because the openings in nozzle-less underdrains are larger than the size of the individual grains of the media, however, it is necessary to use a media support system between the underdrains and the media.
A media support system serves several purposes that are conflicting. For example, very fine media, such as 0.1 to 0.5 mm sand, may be used in potable water type filters. Consequently, a very fine media support is needed to separate this media from the underdrain system and filter bottom and prevent plugging and loss of filter media. Plugging of the underdrain system filter bottom causes a loss of the filtering capacities of the bed and downtime of the filter system. However, large or coarse-pore media support is necessary to promote the formation of larger air bubbles which are desired because they wash a filter better than fine bubbles of air. Jung & Savage, Deep Bed Filtration, Journal American WaterWorks Association, February, 1974, pp. 73-78.
Two types of media support systems have been in common use: (1) support gravel beds comprised of graded gravel placed between the filter media and the filter bottom (or underdrain system) and (2) uniformly porous plates that are anchored to the side walls of the filter or to the underdrain blocks.
When layered gravel beds are used for media support systems, the bed of gravel is usually 12 to 18 inches in height with several layers of varying size gravel. The layers of gravel adjacent to the media and filter bottom are usually coarse and the intermediate layer or layers smaller or finer in size. The finer intermediate gravel layer inhibits the penetration of the media to the underdrain blocks. The coarser gravel in the top or cap layer, however, inhibits plugging of the fine gravel layer. If the finer media penetrates the gravel layers during filtration, it accumulates in the cap layer and is then washed out during the backwash cycle of the filtration process.
U.S. Pat. No. 1,787,689 to Montgomery and U.S. Pat. No. 1,891,061 to Friend et al., for example, disclose a water treating tank containing zeolite water softeners. The gravel beds of the tanks are arranged in an hourglass configuration with layers of coarser and finer gravels.
Gravel layers have several disadvantages including difficulty in installation, the need for deeper filter boxes to allow for the depth of the gravel and higher costs. Also, the gradation of the gravel layers tends to be disturbed during the filtration and backwashing processes and downtime may be required to restore the desired gradation.
Porous plates have been used to replace gravel layers. Porous plates are typically manufactured from sintered plastics. Plastic porous plates, however, are usually buoyant and need to be secured in some way to prevent lifting, especially during the backwash cycle. Prior art methods of securing the porous plate include a combination of screwing and caulking or grouting the plate to the underdrain blocks as disclosed in U.S. Pat. No. 5,149,427 to Brown, or bolting the plate to the underdrain blocks.
U.S. Pat. No. 4,882,053 to Ferri discloses a porous plate used in a filter system without underdrain blocks; the porous plate is attached by a retaining angle secured to each wall of the filter box. The retaining angle holds the plate in place and a seal is made by a sealant bead applied between the side walls and the porous plates.
Problems arise with the above-referenced methods of anchoring the porous plates. Small irregularities in the floor of the filter, the underdrain blocks and the plates can cause seal failures between the plates. Seal failure allows media to penetrate the media support system, causes a progressive failure of the filter underdrain and then of the filter system itself. The underdrains, effluent piping, and clearwell may become plugged with media and the filter bottom may collapse due to excessive pressures which develop during backwash.
U.S. Pat. Nos. 5,149,427 and 5,232,592 to Brown disclose a cap for filter underdrain blocks comprising a porous, planar body. The body of the cap is said to be adapted to support a fine grain filter media without the media penetrating therethrough. The pores in the cap body are approximately 700-800 microns in size.
U.S. Pat. No. 4,882,053 to Ferri, mentioned above, discloses a support or drain plate for filter media comprising porous heat-fusible polyethylene in a traveling bridge filter. The porous drain plates have narrow heat fused, non-porous bands extending vertically through the plates. These bands provide rigidity to the plates said to decrease bowing and subsequent channeling of water during backwash experienced with lap joints. However, the non-porous bands would tend to reduce permeability during filtration and increase head loss.
U.S. Pat. No. 667,005 to Davis discloses a filter bottom for a granular bed that includes three sheets or layers of wire cloth. The upper layer and lower layer are coarse with the intermediate layer being a fine mesh. U.S. Pat. No. 2,267,918 to Hildabolt discloses a porous article formed from metal powders and having plural layers of different porosity. U.S. Pat. No. 5,468,273 to Pevzner et al. discloses a nickel-based filter material having three strata of different porosity used for removing contaminants from air.